Prepare the filling: In a mixing bowl, combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, sliced red onion, and black pepper.

Assemble the quesadilla: Lay one tortilla flat, spoon one-fourth of the filling on one half, and fold over to seal.

Heat the skillet: Preheat a large skillet over medium heat and add olive oil until shimmering.

Cook the quesadilla: Place the filled tortilla in the skillet and cook until golden brown and cheese begins to melt.

Flip and finish cooking: Flip the quesadilla and cook until the other side is golden brown and cheese is fully melted.

Slice and serve: Remove and let cool slightly; slice into wedges and serve warm.